The present disclosure is directed to indoor real-time locating systems (RTLS), methods, and associated applications.
Short-distance, high accuracy ranging is important for many applications such as indoor navigation, body area network (BAN) and unobstructed motion tracking. Meanwhile, recent development in radio-frequency identification (RFID) systems for efficient asset management has made RFID tags and associated devices widely available with low-cost and low-energy consumption. Among RFID ranging methods, received signal strength (RSS) can work over large areas, but suffers from poor accuracy. Time of flight (TOF) is not suitable for short range due to the difficulty in measuring small round-trip time delay. Continuous-wave (CW) phase-based methods using single-tone carriers are preferred for their high accuracy and simple architecture.
However, in indoor environment, passive CW phase ranging by backscattering RFID tags can be challenging. Physically, phase errors caused by multi-path interference are severe in offices and other indoor locales. Poor isolation between transmitting and receiving ends brings strong self-interference that can also smear phase detection. Neighboring readers can also cause strong interference without effective solution by synchronization. Another limitation comes from signal processing. Although CW can be accurate, cycle ambiguity exists when distance is longer than one wavelength. The cycle integer must be solved correctly in order to get high precision. The sensing bandwidth in conventional methods such as DFCW (dual-frequency continuous wave) can be limited by: (1) reading range within which integer ambiguity can be solved or (2) narrowband RFID devices with large phase errors. Although RFID tags with harmonic generation have been proposed, the proposed tags do not address cycle ambiguity.
There is a need for systems and methods for ranging in indoor environment that are accurate, substantially undisturbed by multipath interference and resolve cycle ambiguity.